Nearly forty years ago, it became apparent that household laundry detergents made of branched alkylbenzene sulfonates were gradually polluting rivers and lakes. Solution of the problem led to the manufacture of detergents made of linear alkylbenzene sulfonates (LABS), which were found to biodegrade more rapidly than the branched variety. Today, detergents made of LABS are manufactured worldwide.
LABS are manufactured from linear alkyl benzenes (LAB). The petrochemical industry produces LAB by dehydrogenating linear paraffins to linear olefins and then alkylating benzene with the linear olefins in the presence of HF or a solid alkylation catalyst. The linear paraffins are straight chain (unbranched) or normal paraffins. Normally, the linear paraffins are a mixture of linear paraffins having different carbon numbers. The linear paraffins have generally from about 6 to about 22, preferably from 10 to 15, and more preferably from 10 to 12 or from 11 to 13, carbon atoms per molecule.
LAB processes are described in the book edited by R. A. Meyers entitled “Handbook of Petroleum Refining Processes” (McGraw Hill, N.Y. 1986) and “Ullmann's Encyclopedia of Industrial Chemistry,” Volumes A8 and A13, Fifth Edition (VCH, Weinheim, Germany). Flow schemes are illustrated in U.S. Pat. No. 3,484,498 issued to R. C. Berg, U.S. Pat. No. 3,494,971 issued to E. R. Fenske, U.S. Pat. No. 4,523,048 issued to Vora which teaches use of a selective diolefin hydrogenation zone, and U.S. Pat. No. 5,012,021 issued to B. Vora which teaches use of a selective monoolefin hydrogenation zone. Solid alkylation catalysts are illustrated in U.S. Pat. No. 3,201,487 issued to S. Kovach et al.; U.S. Pat. No. 4,358,628 issued to L. Slaugh; U.S. Pat. No. 4,489,213 issued to S. Kovach; and U.S. Pat. No. 4,673,679 issued to D. Farcasiu. Zeolitic solid alkylation catalysts are disclosed in U.S. Pat. Nos. 3,751,506; 4,387,259; and 4,409,412.
It is well known that aromatic byproducts are formed during the catalytic dehydrogenation of paraffins. For instance, the article starting at page 86 of the Jan. 26, 1970 issue of “Chemical Engineering” states that the product of the dehydrogenation of linear paraffins includes aromatic compounds. The nature of the particular aromatic byproducts that are formed in dehydrogenation is not essential to the operation of the alkylaromatic process. These aromatic byproducts are believed to include, for example, alkylated benzenes, dialkylated benzenes, naphthalenes, other polynuclear aromatics, diphenyl compounds, alkylated polynuclear hydrocarbons in the C.10–C.15 range, indanes, and tetralins, that is, they are aromatics of the same carbon number as the paraffin being dehydrogenated and may be viewed as aromatized normal paraffins. Some aromatic byproducts may be more detrimental than others in deactivating solid alkylation catalysts. It is believed that aromatic byproducts with few or small alkyl groups are more detrimental to solid alkylation catalysts than aromatic byproducts with multiple or long alkyl groups. It is also believed that aromatic byproducts having multiple aromatic rings are more detrimental to solid alkylation catalysts than aromatic byproducts having single aromatic rings. The particular side reactions that lead to the formation of the aromatic byproducts are also not essential to the operation of the alkylaromatic process. An illustration of some of the parallel thermal cracking reactions that can lead to the formation of aromatic byproducts is found in the diagram at the top of page 4–37 of the book mentioned above entitled “Handbook of Petroleum Refining Processes”. Typically, from about 0.2 to about 0.7 weight percent, and generally to the extent of no more than 1 weight percent, of the feed paraffinic compounds to a dehydrogenation zone form aromatic byproducts. Although some commercially available dehydrogenation catalysts are more selective than others at minimizing the formation of aromatic byproducts, it is believed that these byproducts are formed at least to a small extent at suitable dehydrogenation conditions in the presence of most if not all commercially available dehydrogenation catalysts. Since it is an economic advantage to operate the dehydrogenation zone at conditions that produce a high conversion of the feed paraffinic compounds and a high yield of the desired olefins, these aromatic byproducts are produced at least to a small extent in most if not all commercial dehydrogenation zones. But, since these aromatic byproducts have the same number of carbon atoms as both the unconverted feed paraffins and the product olefins, they have boiling points close to that of these paraffins and olefins. Thus, using conventional distillation, the aromatic byproducts are difficult to separate from a mixture such as the dehydrogenation effluent which also contains these paraffins and olefins.
The aromatic byproducts from the dehydrogenation section enter the alkylation section. In the selective alkylation zone containing a solid alkylation catalyst, several possibilities can then occur. First, some of the aromatic byproducts deposit on the surface of the catalyst and as mentioned above deactivate the catalyst. Second, as mentioned above some of the aromatic byproducts are alkylated by monoolefins to form heavy alkylate. Each mole of heavy alkylate formed by this route represents the loss of two moles of feed paraffinic compound toward the production of a less-valuable product and reduces both dehydrogenation selectivity and alkylation selectivity. Third, some of the aromatic byproducts pass through the selective alkylation zone unreacted, are recovered with the overhead liquid stream of the paraffin column which is recycled to the dehydrogenation zone, and ultimately accumulate to unacceptable concentrations. In the prior art processes employing a solid alkylation catalyst, the concentration of aromatic byproducts in the stripping effluent stream can typically accumulate to 4–10 weight percent, which leads to rapid deactivation of solid alkylation catalyst. Where the alkylation catalyst is HF in the prior art processes, the concentration of aromatic byproducts in the stripping effluent stream can typically accumulate to 3–6 weight percent.
Processes for removing the aromatic byproducts that are formed during the catalytic dehydrogenation of paraffins are also known. Suitable aromatics removal zones may be selected from any processing methods which exhibit the primary requirement of selectivity for the aromatic byproducts. Suitable aromatics removal zones include, for example, sorptive separation zones and liquid—liquid extraction zones. See U.S. Pat. No. 5,276,231 and U.S. Pat. No. 5,334,793, the contents of each are incorporated herein by reference. Where the aromatics removal zone is a sorptive separation zone, a fixed bed or a moving bed sorbent system may be used, but the fixed bed system is more common. The sorbent usually comprises a particulate material. In a fixed bed system, the sorbent is typically installed in one or more vessels in a parallel flow arrangement, so that when the sorbent bed in one vessel is spent by the accumulation of the aromatic byproducts thereon, the spent vessel is bypassed while continuing uninterrupted operation through another vessel. A purge stream comprising a purge component, such as C5 or C6 paraffin (e.g., normal pentane), is passed through the spent sorbent bed in the bypassed vessel in order to purge or displace unsorbed components of the stream containing the aromatic byproducts from the void volume between particles of sorbent. After purging, a regenerant or desorbent stream comprising a desorbent component such as C6 or C7 aromatic (e.g., benzene), is passed through the sorbent bed in the bypassed vessel in order to desorb aromatic byproducts from the sorbent. Following regeneration, the sorbent bed in the bypassed vessel is again available for use in sorbing aromatic byproducts.
Thus, a sorptive separation zone for removing the aromatic byproducts typically produces three effluents, which approximately correspond to each of the three steps in the cycle of sorption, purge, and desorption. The composition of each of the three effluents changes during the course of each step. The first effluent, the sorption effluent, contains unsorbed components (i.e., paraffins and olefins) of the stream from which the aromatic byproducts are removed, and also typically contains the desorbent component. With its decreased amount of aromatic byproducts relative to the stream that is passed to the sorptive separation zone, this effluent is used further along in the process to produce alkylaromatics. For example, if the stream that passes to the sorptive separation zone is the dehydrogenation zone effluent, the sorption effluent contains monoolefins and paraffins and thus passes directly to the alkylation zone.
The second effluent, the purging effluent, contains the purge component, unsorbed components of the stream from which the aromatic byproducts were sorbed, and often the desorbent component. The third effluent is the desorption effluent, which contains the desorbent component, the aromatic byproducts, and the purge component. In the typical prior art process, the purging and desorption effluents are separated in two distillation columns. The desorption effluent passes to one column, which produces an overhead stream containing the desorbent and purge components a bottom stream containing the aromatic byproducts which is rejected from the process. The overhead stream of the first column and the purging effluent pass to a second column, which separates the entering hydrocarbons into an overhead stream containing the purge component and a bottom stream containing the desorbent component and unsorbed components of the stream from which the aromatic byproducts are removed. The overhead stream of the second column is used as the purge stream. The bottom stream of the second column is used in the process to produce alkylaromatics. In the example described above where the stream that passes to the sorptive separation zone is the dehydrogenation zone effluent, the bottom stream of the second column contains benzene, monoolefins, and paraffins and flows directly to the alkylation zone.
This two-column process for separating the purge and desorption effluents wastes energy. Energy is consumed to reboil the desorbent component (e.g., benzene) in the first column, to reboil the purge component (e.g., n-pentane) in the second column, and to heat the desorbent component in the second column. This process also has a high capital cost because two columns are needed. Thus, a process is sought in which the streams containing the aromatic byproducts, purge component, and desorbent component are produced in a more efficient manner that uses fewer utilities than the prior art two-column process.
Distillation columns have, of course, been widely used to perform many separations in industry. Over fifty years ago, Wright proposed replacing two distillation columns with a single distillation column having a vertical partition (dividing wall column) within the column that would effect the separation of the column feed into three constituent fractions. It was recognized then that a dividing wall column could minimize the size or cost of the equipment needed to produce overhead, bottoms, and sidedraw products. See U.S. Pat. No. 2,471,134 (Wright). Wright described using the dividing wall column to separate a mixture of methane, ethane, propane, butanes, and a small amount of C5 and heavier hydrocarbons. Since then, researchers have studied the dividing wall column and have proposed using dividing wall columns for separating other mixtures, including xylenes (Int. Chem. Engg., Vol. 5, No. 3, July 1965, 555–561); butanes and butenes (See e.g., Trans IChemE, Vol. 70, Part A, March 1992, 118–132); methanol, isopropanol, and butanol (See e.g., Trans IChemE, Vol. 72, Part A, September 1994, 639–644); ethanol, propanol, and butanol (Ind. Eng. Chem. Res. 1995, 34, 2094–2103); air (See e.g., Ind. Eng. Chem. Res. 1996, 35, pages 1059–1071); natural gas liquids (Chem. Engg., July 1997, 72–76); and benzene, toluene, and ortho-xylene (Paper No. 34 K, by M. Serra et al., prepared for presentation at the AIChE Meeting, Los Angeles, Calif., USA, November 1997). The Serra et al. paper also describes separating mixtures of butanes and pentane; pentanes, hexane, and heptane; and propane and butanes.
Control systems for dividing wall distillation columns have been known since at least 1980, when U.S. Pat. No. 4,230,533 issued to Giroux describing a dividing wall distillation column and its control system. In the late 1990's, control systems for dividing wall distillation columns have been studied in further detail by researchers. For example, the separation of a ternary mixture consisting of methanol, iso-propanol, and butanol into three products using a dividing wall distillation column has been described in the article in Trans IChem, Vol. 76, Part A, March 1998, 319, by M. I. Abdul Mutalib et al. The article describes a control configuration in which the top reflux and the middle reflux are used as manipulated variables, while keeping the reboiler vapor constant. The article also mentions using temperatures above the top of the dividing wall, between the top and the bottom of the dividing wall, and below the bottom of the dividing wall for control purposes. According to the article, a Ph. D. thesis by M. I. Abdul Mutalib (UMIST, Manchester, UK, 1995) entitled “Operation and Control of the Dividing Wall Column” also describes such a configuration.
Despite the advantages of the dividing wall column and despite much research and study, the processing industry has long felt reluctant to use dividing wall columns in commercial processes. This widespread reluctance has been attributed to various concerns, including control problems, operational problems, complexity, simulation difficulties, and lack of design experience. See, for example, the articles by C. Triantafyllou and R. Smith in Trans IChemE, Vol. 70, Part A, March 1992, 118–132; F. Lestak and C. Collins in Chem. Engg., July 1997, 72–76; and G. Duennebier and C. Pantelides in Ind. Eng. Chem. Res. 1999, 38, 162–176. The article by Lestak and Collins sets forth some general guidelines and considerations when substituting a dividing wall column for conventional columns. Nevertheless, the literature documents relatively few practical uses of dividing wall columns in commercial plants. See the article by H. Rudd in The Chemical Engineer, Distillation Supplement, Aug. 27, 1992, s14–s15 and the article in European Chemical News, Oct. 2–8, 1995, 26.
Prior art alkylaromatic processes, in particular, do not use dividing wall distillation columns. Nor do they use fully or non-fully thermally coupled distillation columns, which, as explained in the above-mentioned article by C. Triantafyllou and R. Smith, are thermodynamically equivalent to dividing wall columns when there is no heat transfer across the dividing wall. In particular, a dividing wall distillation column has not been used for separating the effluent streams from a sorptive separation step in an alkylaromatic process. This is not only for the reasons given above but also for three additional reasons. First, the focus of prior research studies has been on separating relatively unchanging mixtures of only a few (e.g., 3 to 5) components, whereas the purging effluent contains dozens of compounds and its composition changes gradually yet significantly from the start to the end of the purging step. In addition, the desorbent effluent likewise contains dozens of compounds, and its composition also changes to a significant extent over the course of the desorption step. Second, the research studies produce dividing wall distillation product streams in which co-boiling components are recovered in the same stream, whereas the separation of the purging and desorption effluents preferably produces the aromatic byproducts in one stream, so that they can be rejected from the process, and the monoolefins and paraffins in another stream for further use in the process. Third, achieving a commercially-useful long life of the solid alkylation catalysts used for the production of LAB requires that the composition of the stream containing the desorbent component be controlled relatively tightly, since the presence of aromatic byproducts in this stream tends to rapidly deactivate solid alkylation catalysts. Thus, alkylaromatic processes are characterized by changing compositions of the purging and desorbent effluents, unique requirements for the separation of co-boiling compounds, and a relatively tight specification on the aromatic byproducts in the stream containing the desorbent compounds. This combination compounds the problems, difficulties, and complexity of using a dividing wall distillation column or two thermally coupled distillation columns. In addition, in order for any such distillation column arrangement to be adopted commercially by the processing industry, a dependable control system is required.